This invention relates to a radiation image storage panel having a stimulable phosphor layer, in particular, a radiation image storage panel that can provide radiation images which are high in radiation sensitivity and sharpness and a method for preparing the same.
Radiation images like X-ray images are often used in diagnosis of diseases. For obtaining the X-ray images, there has been employed so-called radiation photography in which X-ray transmitted through a subject is irradiated on a phosphor layer (fluorescent screen), visible light thereby generated is irradiated on a film using silver salt in the same manner as in conventional photography, and the film is then developed. However, in recent years, methods of directly taking out images from phosphor layer have been devised to replace methods using film coated with silver salts.
The methods include, for example, a method in which the radiation (generally X-ray) transmitted through a subject is absorbed by a phosphor, and thereafter this phosphor is excited by light or heat energy to bring the radiation energy accumulated by being absorbed as mentioned above to radiate as fluorescence, which fluorescence is detected and formed into an image. Specifically, U.S. Pat. No. 3,859,527 and Japanese Unexamined Patent Publication No. 12144/1980 disclose radiation image storage methods in which a stimulable phosphor is used and visible light or infrared rays are used as stimulating light. This method employs a radiation image storage panel (hereinafter referred to as "storage panel") comprising a support formed thereon with a stimulable phosphor layer (hereinafter referred to simply as "stimulable layer"), where radiation transmitted through a subject is applied to the stimulable layer to accumulate radiation energy corresponding to the radiation transmission degree of all areas of the subject to form a latent image. Thereafter this stimulable layer is scanned with the stimulating light to bring the radiation energy accumulated in the areas to radiate to convert this into light, thus obtaining an image according to signals based on the strength of this light. The image finally obtained may be reproduced as a hard copy, or may be reproduced on a CRT.
The storage panel having the stimulable layer used in this radiation image storage method is required to provide images with good graininess and high sharpness, in addition to high precision in radiation absorption and light conversion (including both, herein called "radiation sensitivity").
However, in general, the storage panels having the stimulable layer are prepared by coating on a support or a protective layer a dispersion containing a particulate stimulable phosphor having a particle diameter of about 1 to 30 .mu.m and an organic binder, followed by drying, resulting in a low charge density of the stimulable phosphor (charge weight: 50%), so that the phosphor layer must be made to have a large thickness to achieve sufficiently high radiation sensitivity.
According to one example, when the layer thickness of the stimulable phosphor layer is 200 .mu.m, the coating amount of the stimulable phosphor is 50 mg/cm.sup.2. The radiation sensitivity is increased directly until the layer thickness of the stimulable phosphor layer becomes 350 .mu.m, and is saturated above 450 .mu.m.
In this regard, the reason why the radiation sensitivity is saturated is that stimulated emission inside the stimulable phosphor layer does not pass outside, because the scattering of the stimulated emission between the stimulable phosphor particles occurs when the stimulable phosphor layer becomes excessively thick.
On the other hand, the sharpness of the image in the above radiation image storage method has a tendency to be higher with decreased thickness of the stimulable layer of a storage panel, so that the phosphor layer must be made thinner to improve the sharpness.
Also, the graininess of the image in the above radiation image storage method depends on the spatial fluctuation of radiation quantum number (i.e. quantum mottles) or the structural disorder of the stimulable layer of a storage panel (i.e. structure mottles), so that making the layer thickness of the stimulable layer smaller brings about a decrease in the radiation quantum number to be absorbed in the stimulable layer to increase the quantum mottles. Alternatively, structural disorders are actualized to cause a lowering of image quality. Thus, the phosphor layer must be made to have a large thickness to improve the graininess of images.
In other words, in the conventional panels, the sensitivity to radiation and the graininess of images show quite opposite tendencies to the sharpness of images with respect to the layer thickness of the stimulable layer. Accordingly, it has been necessary to balance sensitivity to radiation against graininess and sharpness of the image.
Incidentally, as well known the sharpness of images in the conventional radiography depends on the extent of the momentary emission (emission at the time of irradiation of radiations) of the phosphor present in a fluorescent screen. In contrast therewith, however, the sharpness of images in the radiation image storage method utilizing the above-mentioned stimulable phosphor does not depend on the extent of stimulated emission of the stimulable phosphor present in the storage panel, namely, it does not depend on the extent of the emission of the phosphor as shown in the radiography method, but depends on the extent of the stimulating light in said storage panel. The reason why is as follows. In the radiation image storage method, the radiation images accumulated in the storage panel are taken in time series, and, accordingly, all of the stimulated emission generated by the stimulating light irradiated in a certain period (ti) are desirably collected and recorded as an output from a certain picture element (xi, yi) on the storage panel from which the stimulating light was irradiated. However, if the stimulating light is extended in said panel due to, for example, scattering, etc., the extended stimulating light also stimulates the stimulable phosphors present outside the irradiated picture element (xi, yi), so that recorded information output from the picture element (xi, yi) include outputs from a broader area than the picture element (xi, yi). Therefore, if the stimulated emission generated by the stimulating light irradiated at a certain time (ti) is comprised only of the emission from picture elements (xi, yi) on said panel on which the stimulating light had been actually irradiated at the time (ti), the emission, whatever extent it has, does not affect the sharpness of the resulting image.
From such a viewpoint, there have been proposed several methods for improving the sharpness of radiation images.
They are exemplified by a method in which a white powder is mixed into the stimulable phosphor layer of a radiation image storage panel as described in Japanese Unexamined Patent Publication No. 146447/1980, and a method in which a radiation image storage panel is so colored that the average reflectance at the stimulating excitation wavelength region of a stimulable phosphor may become smaller than the average reflectance at the stimulated emission wavelength region of said stimulable phosphor as described in Japanese Unexamined Patent Publication No. 163500/1980.
These methods, however, necessarily result in an extreme lowering of the sensitivity if the sharpness is improved, and can not be said to be preferable methods.
On the contrary, the present inventors have already proposed, in Japanese Unexamined Patent Publication No. 73100/1986, a novel storage panel containing no binder by which the above-mentioned conventional disadvantages in the storage panels using the stimulable phosphors are improved. According thereto, the stimulable layer of the above storage panel contains no binder, so that the charge ratio of the stimulable phosphor in the stimulable layer can be remarkably improved and also the directivity of the stimulating light and stimulated emission of the stimulable layer can be improved, resulting in improvement in the sensitivity to radiation of the storage panel and the graininess of images, and at the same time improvement in the sharpness of images.
The stimulable layer of the storage panels proposed in the above-mentioned publications can be prepared by the vapor phase build-up methods such as vapor deposition and sputtering.
Performances necessary for a support of the storage panel are excellent mechanical properties and planarity or a property maintaining a rigidly planar state, chemically inert and opaque to the stimulating light and the stimulated emission. Further, in the case of forming the phosphor layer by use of vapor-phase build-up method, heat-resistance is important for preventing deformation due to heating which is applied during vapor phase build-up or annealing treatment after the vapor phase build-up. The opaqueness of the support is important, because, if the support is transparent, incident stimulating light or stimulated emission transmit through and pass outside from the support to lower the radiation sensitivity. Materials conventionally used as a support, for instance, those disclosed in Japanese Unexamined Patent Publication No. 99900/1986, such as plastic films, e.g., cellulose acetate film, polyester films and polyethyleneterephthalate film and papers, e.g., photographic base paper, resin-coated paper, barayta paper etc. are inferior in heat-resistance. Also, though quartz and chemical reinforced glasses have good planarity and also relatively excellent heat-resistance, they have high transparency, and are resultingly inappropriate to use as it is. Accordingly, those preferably used as the preferable support have been limited to opaque crystallized glasses and ceramic plates.
The present inventors have further proposed, in Japanese Unexamined Patent Publication No. 142497/1986, a storage panel having, on a support having a large number of fine concave-convex pattern on its surface, a stimulable layer comprising a fine pillar-shaped block structure which is grown on concave portions of the pattern. The pillar-shaped block stucture of crystals grown on the concave portions can be prepared by forming fine concave-convex pattern on the support by use of printing method, photography etching method, etc., and providing a stimulable layer on the support by applying preferably vapor phase build-up methods.
FIG. 3 is a cross-sectional view of an example of the storage panel which is a disclosed technique in the above-mentioned publication. In FIG. 3, the numeral 10 denotes a storage panel; 12, a support; 13, a stimulable layer; 11ij, a convex portion; 11ij', a concave portion; 13ij, a fine pillar-shaped stimulable phosphor grown on the convex portion; and 13ij', a fine pillar-shaped stimulable phosphor grown on the concave portion. FIG. 4(a), 4(b) and 4(c) are respectively plane-views of several examples of the concave-convex pattern of the support surface. Incidentally, in FIGS. 4(a) to 4(c), 21ij is a convex portion protruded on partitioned areas on a support 22. 21ij' is a concave portion corresponding to said convex portion. The success of the fine concave-convex pattern depends on the height of the convex portion and the ratio of area of the concave portion to the convex portion.
The present inventors also have further proposed in Japanese Unexamined Patent Publication No. 142498/1986, a storage panel having, on a support having a large number of fine concave-convex pattern on it surface, a stimulable layer comprising a fine pillar-shaped block structure which is grown on a convex portions of the pattern.
In the storage panel having the concave-convex pattern, scattering of the stimulating light in the stimulable layer is remarkably reduced and the sharpness of the image is enhanced. Accordingly, the technique for forming the fine concave-convex pattern on the surfaces of the above-mentioned crystallized glasses, ceramics, metals, etc. is important. However, to provide evenly a fine concave-convex pattern on a large ares is technically difficult, because each of the above methods for forming the concave-convex pattern is accompanied with complicated steps and high cost.